So don't get too used to it if you're not going to upgrade your operating system.

Of course if you rushed to download the beta Messages app immediately then you'll probably buy OS X 10.8 - expected to hit the shelves in July 2012. If it's priced the same as previous Mac OS X upgrades, it will cost £20.99 ($29) to trade up from Mac OS X Lion.

A French blogger spotted the beta app's expiration date in the software code of the download:

Thank you for participating in the Messages Beta program. With the inclusion of Messages in OS X Mountain Lion, the Messages Beta program has ended. To continue using Messages, please visit the Mac App Store and purchase OS X Mountain Lion.

Though the linked-up cross-device messaging service is one of the big assets of the updated OS, some of the software's test drivers reckon that linking iMessages into Mac's instant chat on the desktop has resulted in a waterfall of instant communication. Even for some fanbois, it's been too much.

An instant message sent by your iBuddy will ping on your iPhone, your Mac and your iPad, if you have left those devices on. What's more, it will keep pinging away until you have explicitly opened and received the notes on each device individually.

And with the increased speed of chatting, made possible by having the platform available to Mac users with proper keyboards, Mac-to-Mac instant chat will drive your iPad bonkers with unread messages.

Also if you leave your iPhone on overnight, it will appear that you're always available on instant chat. So where texting etiquette would stop you dinging your friend's phone late at night, a chatty pal with a Mac could just assume you're up on the computer and hammer out five IMs before clocking that you're actually asleep - or were, anyway. The joys of beta software testing. ®